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Dark Energy Made an Early Entry

Dark energy--the mysterious stuff that is stretching space and speeding the expansion of the universe--has been around at least 9 billion years, a team of astronomers and astrophysicists announced today at a NASA teleconference. The new findings should help scientists narrow down possible explanations for just what the stuff is.

Since the 1920s, scientists have known that the universe is expanding in all directions. In 1998, two teams of astronomers and astrophysicists stunned their colleagues when they reported that, contrary to all expectations, the expansion is speeding up. This came as a surprise, as researchers expected the universe's expansion to slow as galaxies tug on one another with their gravity. The observation showed that some sort of "dark energy" is stretching space, blowing it up like a balloon.

Still, no one can explain precisely what dark energy is. It could be a kind of incessant pressure woven into the fabric of space itself, a concept known as a cosmological constant and dreamt up by Einstein. If so, then the amount of dark energy would grow with expanding space so that each cubic centimeter always contains the same constant amount. Alternatively, dark energy could be some new sort of field, a bit like an electric field, that fills space and grows weaker and more dilute as space expands. Further complicating matters, no one knows if the behavior of dark energy has changed over the 13.7-billion-year history of the universe.

As in the previous studies, astrophysicist Adam Riess of Johns Hopkins University and the Space Telescope Science Institute in Baltimore, Maryland, astronomer Louis-Gregory Strolger of the University of Western Kentucky in Bowling Green, and colleagues studied stellar explosions known as type Ia supernovae. These celestial firecrackers serve as standard candles, meaning that they all pump out roughly the same amount of light. That allows researchers to determine how far away a supernova is by how bright it appears in the sky. At the same time, the expansion of the universe stretches the light from the explosion to longer and redder wavelengths. So by tracking this "red shift" for supernovae at various distances, researchers can reconstruct the history of the expansion of the universe.

Over the past several years Riess and Strolger's team used the Hubble Space Telescope to spot 23 extremely distant supernovae that exploded between 8 billion and 10 billion years ago. Studying the red shift-distance relationship for these supernovae, the researchers determined that dark energy must have existed even back then. Although the experimental uncertainties are big, the data fit with the notion that the dark energy is a cosmological constant, Riess reports. Previous observations of closer supernovae are also consistent with a cosmological constant.

"This set of observations is ruling out the idea that dark energy may have changed recently," says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena. But the observations cannot yet distinguish between a cosmological constant and some sort of field. To do that will take hundreds of supernovae, says Saul Perlmutter an astrophysicist at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, most likely spotted by a specially designed space mission.